Conventionally, coal-fired furnaces for fossil fuel power plants require coal and air as their input. In the furnace, the coal is burned and creates an exhaust, which comprises a gas and particulate. The gas portion of the exhaust is commonly composed of about 78% nitrogen, about 15% carbon dioxide, about 3% oxygen, and about 4% various oxides made during combustion. The particulate portion of the exhaust comprises ash, which includes minerals other than carbon that will not combust into a gas form, and soot, which is unburned carbon or carbon formed from incomplete combustion. As the exhaust exits the furnace, most of the gas is invisible while most of the particulate is visible. Almost all coal-fired power plants are required under state law to capture most of the visible particulates.
In many coal-fired power generation plants, the primary device used to capture the particulates is called an electrostatic precipitator. In operation, the electrostatic precipitator provides a large negative electric field on wires or rods, and this field negatively charges the ash and soot particles. The negatively-charged particles are then electrostatically attracted to grounded or positively-charged plates in the precipitator, commonly called collector plates. As the negatively-charged particles travel through the precipitator, they magnetically attach to the grounded or positively-charged collector plates. Eventually, these particles can be collected in hoppers for landfill disposal, rather than being released into the atmosphere. In operation, the precipitator prevents large black plumes from exiting stacks of a power plant.
Mercury can enter the furnace by piggybacking on the coal. Hence, the exhaust gas can contain a small percentage of mercury. For example, the amount of mercury is about one to two pounds in about 10,000 tons of coal. The heat of the furnace transforms the mercury to its gaseous state, which is not visible due to its concentration level in the parts per trillion. Recently passed laws and regulations, e.g. the Utility MACT, have placed strict requirements on the amount of mercury that can be emitted into the atmosphere from the boilers of coal-fired electric power generation plants, as well as other industrial plants. Thus, power generation companies have invested billions of dollars in developing new technologies for capturing mercury, and other unwanted species, in the flue gases emitted from power plants.
One such technology involves injecting finely ground PAC into the exhaust from the boiler once the exhaust passes through the precipitator. PAC is a sorbent and can adsorb and absorb a majority of mercury that would otherwise be exhausted into the atmosphere. After the PAC is injected into the exhaust, the exhaust is passed through a baghouse that serves as a large filter to remove the PAC, thus removing a portion of the mercury. Conventionally, injection rates of about 1.5 to about 5 pounds of PAC per million cubic feet per minute of furnace gas are needed to control approximately 90% of the mercury output.
Unfortunately, the amount of mercury removed from the exhaust is not linearly related to the amount of PAC injected into the exhaust. Specifically, increasing the amount of PAC injected into the exhaust does not necessarily lead to the same increase in the amount of mercury removed from the exhaust. This phenomenon is thought to be a result of the complex mechanisms involved in adsorption. It is essential that the PAC remain in contact with the flue gas for a sufficient amount of time to increase the probability of capture, and thus increase the portion of the unwanted species removed. The PAC, however, cannot remain in the flue gas beyond the time where its affective adsorption capacity has been spent. For this reason one must have close control of the dust cake formation and removal within the filter device.
Therefore, there is a desire for systems and methods for controlling the composition of the exhaust/PAC mixture to optimize the dust cake control within the filter. Various embodiments of the present invention address such a desire.